1. Field of the Invention
This invention relates to soil and groundwater remediation, and in particular the in situ removal of halogenated hydrocarbons and other organohalides as well as Cr(VI) that contaminate soil formations and the fluids present in the formations.
2. Description of the Prior Art
Concern about the contamination of soils and groundwater is continually increasing owing to the effect of the contaminants on the quality of life and the environment, and the limitations that they impose on the use of land and on the conversion of unused or underutilized land sites to useful purposes. Fuel dumps, military waste sites, landfills, and other hazardous waste sites in general are among those that are contaminated, and halogenated organic compounds and Cr(VI) are among the most prominent of the contaminants. A source of Cr(VI) is dichromate, which was formerly in common use as a cleaning agent among chemistry laboratories and as a result is now a common contaminant at laboratory sites. Many halogenated organic compounds are known carcinogens, mutagens or both, forming underground plumes that migrate into water wells and aquifers, while Cr(VI) has been implicated in tubular necrosis of the kidney, allergic reactions and dermatitis, and reproductive toxicity, and is a possible carcinogen and mutagen. The removal of these contaminants is considered critical to the remediation of soil and groundwater.
Groundwater can be removed from the soil and treated aboveground for decontamination prior to use, and soil can likewise be excavated and treated aboveground. The more preferred methods of remediation, however, are those that are performed in the soil formation itself (i.e., "in situ"), because these methods offer a lower cost, less disruption to the environment, and less exposure of workers to the contaminants, than above-ground treatment methods. Effective remediation is not always achieved by in situ methods, however. One in situ method is microbial degradation, either biotic or abiotic. Unfortunately, many halogenated organic compounds are not rapidly metabolized by bacteria. For this reason, methods of promoting reductive dehydrohalogenation by the bacteria have been attempted. Unfortunately, the products of reductive dehydrohalogenation are often toxic as well, and in some cases more so than the original contaminants. Another in situ method is the use of electrokinetics, in which an electric current is passed through the soil to cause the water to migrate due to electroosmosis. The migration draws the water from one layer of the formation into a different layer where decontamination is supposedly easier to achieve, but the flow rate is very slow and the process still requires a method for the decontamination. A third in situ method is soil-vapor extraction, in which the contaminants are drawn as vapors from the vadose section of the soil formation above the water table. Contaminants dissolved in the groundwater or in organic matter then vaporize to replenish the contaminants that were removed from the vapor in a continuous vaporization and extraction process. The vaporization step is a limiting step, however, and the process requires drawing a vacuum on the soil formation and maintaining air flow through the formation toward the extraction point(s). Furthermore, workers above ground risk exposure to the contaminants as they reach the surface.
A fourth in situ method that has been disclosed is reductive dehalogenation by the use of metals in the soil. Magnesium has been cited as a metal for possible use in this type of process. A disclosure of the process appears in Seech et al., U.S. Pat. No. 5,480,579 (Jan. 2, 1996). The difficulty with processes of this type is that the magnesium metal surface becomes contaminated with oxidized magnesium compounds in which the magnesium is no longer functional in the dehalogenation reaction owing to its oxidized state and which block access to the reactive magnesium metal, thereby greatly decreasing the rate and extent of further dehalogenation.